It is noted that it is nearly impossible to locate antennas on airborne platforms that have a perfect 360° field of view. Usually there is a close obstruction or scatterer in a particular direction that prevents the antenna from seeing around it. A shadow related to the blockage width is cast upon the pattern of the antenna along the direction of the obstruction. The result is a shadow area to the far side of the obstruction that blocks passage of RF energy, thus preventing the transmission or receipt of signals in that direction.
Adding extra antennas to cover these poorly illuminated areas is usually not an option due to the added weight of the antenna and cabling, as well as switching accessories, air drag, added cosite interference problems or simply the lack of room for another antenna.
There is therefore a need for providing a mechanism to mitigate the effects of scattering due to the obstruction and more particularly the pattern blockage so that a true 360° field of view coverage is achievable.
It is noted that an antenna emits spherical wave fields that are expanding away from the antenna. Monopole or blade-like antennas on a conductive surface radiate in a vertically polarized fashion such that a vertically polarized signal is emitted normal to the ground plane. Between the antenna and the obstruction are the near-field and perhaps including the Fresnel zone in which a free space wave and surface wave would expand radially producing a circular isophase front. The result is that the wave front of waves from the antenna impinges upon the obstruction in an arcuate or circular fashion.
The result of the impingement of an arcuate wave front on an obstruction in which the obstacle is in the near field of the antenna, is that a large shadow is created behind the object. This phenomenon is a result of Fresnel defraction.
When an obstacle is in the far field of the radiating antenna, the local field around the obstacle has a nearly equi-phase wavefront and is called a plane wave. The field blockage caused by the obstacle is a small percentage of the overall effective plane wave aperture around the obstacle. Hence blockage effects which are manifested by deep nulls in the radiation pattern are minimized.
However, absent any wave front reconfiguration when the obstacle is close to the radiating antenna i.e. within a few wavelengths, the field front is radial and is not a plane wave. What this means is that the wave front of the energy impinging upon the obstacle in the near field is curved, with the resulting defraction at the obstacle providing a wider swath or shadow behind the obstacle. This is because the area behind the obstacle is not filled in either close to the obstacle or at considerable distances. The result is that the obstacle blocks a significant amount of the radiating signal along its illuminating path line and to either side thereof extending the shadow region deeply into the far field.
In the past antenna engineers have tried to minimize the blockage of an obstacle by placing layers of dielectric materials around the obstacle to force “creeping” of the wave to flow around the object to fill and/or illuminate the shadow cast by the blockage. For complex obstacle shapes, placing of materials of appropriate thickness and orientation on the object is impractical.
Oftentimes antenna engineers will place radar absorbing material or other absorbing materials on the obstacle just to minimize the undesirable field defracting around the edges of the obstacle. However, the result is a reduction in the gain along the direction of the obstacle.
An additional problem with close obstructions is that they can reflect strong signals back to the antenna and beyond. If these reflections are out of phase, deep nulls in the antenna pattern may occur in the reverse direction.